Scanning Microwave Microscopy is a technique for measuring dielectric properties of a substrate surface on a nanometer scale. The technique involves establishing a quasi-static electromagnetic wave between the tip of a scanning probe and the substrate surface. Then, as the tip of the scanning probe is scanned over the substrate surface, the impedance of a resonant structure formed between the tip of the scanning probe and the substrate surface is measured. The impedance is typically measured at one or more microwave frequencies using a network analyzer or other suitable measurement instrument that is coupled to the resonant structure via a signal cable. However, as the tip of the scanning probe is scanned over the substrate surface, there is mechanical deformation of the signal cable due to the resulting displacements of the scanning probe relative to the measurement instrument. These mechanical deformations result in corresponding changes of the characteristic impedance of the signal cable, which may degrade the impedance measurements of the resonant structure that are acquired by the measurement instrument.
In prior art signal cables, the mechanical deformation occurs at localized positions of the signal cable. This type of mechanical deformation causes changes in characteristic impedance of the signal cable that are typically not repeatable or predictable, which makes it impractical to calibrate out the effects of the mechanical deformation. The changes in characteristic impedance of the signal cable due to this type of mechanical deformation are also difficult to distinguish from changes in impedance of the resonant structure that are measured as a result of scanning the tip over the substrate surface. This makes it impractical to filter out the effects of the mechanical deformation of the signal cable from the measurements of the impedance of the resonant structure that are acquired by the measurement instrument.
In view of the above, there is a need for an improved signal coupling system between a measurement instrument and a scanning probe that is suitable for use in Scanning Microwave Microscopy.